Numerous fibrous filtration sheets have been prepared from a variety of fibers, such as glass, polyethylene, polypropylene, polyester, and aramid fibers. While these sheets are suitable for many applications, particularly many filtration applications, they are generally incapable of reliably removing small micron and submicron particulate matter and bacteria and viruses. While the thickness of such a fibrous sheet can be increased to compensate for the relatively large pore size of the sheet, the flow resistance of the sheet then becomes impracticably high. This inability to prepare fibrous filtration sheets of sufficiently small pore size is generally the result of the unavailability of fibers of sufficiently fine diameter, inasmuch as the pore size of a fibrous filtration sheet is related to some extent to the fiber diameter used to prepare such a sheet, i.e., uniformly smaller diameter fibers will more easily allow for the preparation of a fibrous filtration sheet of uniformly smaller pore size.
While membranes, such as microporous and ultrafiltration membranes, are capable of providing the desirable smaller pore size as compared to fibrous nonwoven webs, membranes suffer from many performance disadvantages as compared to fibrous nonwoven webs. For example, membranes are relatively brittle and require considerable care in handling. As a result, membranes are difficult to convert into corrugated structures for filter elements without introducing cracks in the membranes, particularly in preparing small radii filter elements, with the consequent loss of filtration efficiency. The relative fragility of membranes also render membranes susceptible to failure upon pulsing of the fluid flow through the membranes.
Accordingly, there remains a need for a fibrous nonwoven web with a relatively small pore size, preferably sufficiently small to remove submicron particles, especially bacteria and viruses, at a reasonable thickness and/or flow resistance through the fibrous nonwoven web. Such a fibrous nonwoven web preferably also would have a high modulus, as well as sufficient elongation, so as to withstand filtration conditions, such as pulsing, which can be encountered in commercial filtration processes. The handling characteristics of such a fibrous nonwoven web also should be such that the fibrous nonwoven web can be converted into a corrugated structure for a filter element without damaging the web in any way so as to affect filtration efficiency. A high temperature resistance and good chemical stability are other useful and desirable characteristics of such a fibrous nonwoven web.
The present invention provides such a fibrous nonwoven web, as well as a method of preparing and of using such a fibrous nonwoven web, particularly in filtration applications. These and other objects and advantages of the present invention, as well as additional inventive features, will be apparent from the description of the invention provided herein.